1. Field of the Invention
The present invention relates to a method for manufacturing organic pigments, an electrophotographic photoconductor using the organic pigments, and an electrophotographic image forming method and apparatus using the electrophotographic photoconductor.
2. Discussion of the Related Art
Organic pigments have been used as a filler for paints because a variety of color paints can be prepared by using them, which is an advantage over inorganic pigments. In recent years, attention is focused on organic pigments because of being able to be used as an organic photoelectric conversion material, and therefore various organic pigments have been developed.
When a layer including such an organic pigment is formed, a wet film-forming method is typically used because a layer of large size can be easily formed. As for the wet film-forming method, a method is typically used in which a coating dispersion including an organic pigment is coated on a substrate and dried to form a coating layer on the substrate. The coating properties of the coating layer formed by a wet film-forming method depend on whether the pigment is uniformly dispersed in a vehicle of the coating dispersion during the coating and drying process.
In order to prepare a good coating dispersion in which a pigment is uniformly dispersed, various dispersing devices and systems have been proposed. In addition, various methods for improving dispersion efficiency have also been proposed. When it is desired to prepare a coating dispersion in which pigment particles having a small particle diameter are uniformly dispersed, a dispersing medium (such as balls used for ball milling methods) of small size is typically used. However, even when a small size dispersing medium is used, a good coating dispersion cannot be necessarily prepared if the pigment used has a property such that it is not easily dispersed in the vehicle used. In order to improve the dispersing property of such a coating dispersion, there are two methods in which a large size dispersing medium is used and a pigment which can be easily dispersed is used. When the former method is used, the particle size of the pigment in the resultant dispersion is relatively large compared to that of the pigment in a coating dispersion dispersed by a small size dispersing medium. Therefore, it is preferable to use the latter method. However, it is difficult to design and synthesize a pigment which can be easily dispersed because it is hard to grasp the relationship between the physical and chemical properties of a pigment and the dispersing property of the pigment in a coating dispersion.
On the other hand, various information processing systems using electrophotography have been developed in recent years. In particular, photo printers in which information, which is converted to digital signals, is recorded in a photosensitive material using light have been dramatically improved in recording qualities and reliability. This digital recording technique is applied not only for printers, but also for copiers, and so-called digital copiers are developed. Since digital copiers have more information processing functions than analogue copiers, it is supposed that the demands for digital copiers will increase more and more from now on.
Laser diodes (LDs) and light emitting diodes (LEDs) are typically used as a light source of photo copiers and printers because of being small in size and having good reliability and low manufacturing cost. As for LEDs, an LED emitting light of 660 nm in wavelength is typically used. As for LDs, an LD emitting near infrared light is typically used. Therefore, a need exists for photoconductor having high photosensitivities over a wavelength range of from the visible region to the near-infrared region.
The photosensitivity of an electrophotographic photoconductor almost depends on the photosensitivity of an electron generating material used in the photoconductor. As for charge generating materials, various kinds of materials such as azo type pigments, polycyclic quinone type pigments, trigonal system selenium, phthalocyanine pigments and the like have been developed. Among these pigments, titanyl phthalocyanine (hereinafter referred to as TiOPc) is very useful for a photoconductor for image forming apparatus such as printers and copiers, in which an LED or LD is used as a light source, because of being sensitive to light having a wavelength of from 600 to 800 nm.
In addition, a photoconductor used for electrophotography such as Carson process and the like is required to have the following charge properties as well as the high sensitivity property to the specific light mentioned above:
(1) a good charging ability in which a high electric potential can be formed and maintained when a photoconductor is charged;
(2) a good charge decaying ability in which when a photoconductor is exposed to light, the electric potential previously formed on the photoconductor rapidly decays and the residual potential is low; and
(3) a good charge stability in which a photoconductor can maintain a good charging ability and a good charge decaying ability even when the photoconductor is used for a long time.
In particular, in high sensitive photoconductors such as photoconductors including TiOPc, the charging ability thereof tends to deteriorate and the residual potential tends to increase when the photoconductors are repeatedly used.
Because of these reasons, a need exists for a photoconductor including TiOPc and having a good charge stability.